Short length tapered extrusion cooking apparatus having peripheral die

ABSTRACT

An improved short-length, high-speed extrusion device (10) is provided with having increased capacities and the capability of producing high quality extrudates with improved density and pellet integrity properties. The device (10) includes an extruder (14) equipped with a peripheral die (20,118), the latter having die outlet openings (100,120) oriented generally transverse to the longitudinal axis of the extruder barrel (16). The die (20) has a plurality of axially spaced series (92-98) of openings (100), wherein each series (92-98) has a different number of openings (100). Alternately, the die (118) is of radially enlarged design relative to the extruder barrel (16) and has only a single series of openings (120). In both instances, an internal flow-directing bullet (108,140) is positioned within the corresponding die (20,118) in order to smoothly direct flow of product from the outlet (21) of the barrel (16) to the transversely oriented die outlet openings (100,120).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present is broadly concerned with improved short length, high speedextrusion cooking devices of the type generally described in U.S. Pat.No. 5,694,833 equipped with peripheral dies allowing greater productthroughputs while increasing product densities, cook values and moisturelevels. More particularly, the invention pertains to such improvedextrusion cookers, and corresponding methods, wherein a peripheralextrusion die is disposed across the extruder barrel outlet and has aseries of extrusion outlet openings oriented transverse to thelongitudinal axis of the barrel. The peripheral dies of the inventionmay be provided with individual, axially spaced apart series of dieoutlet openings (usually with different numbers of outlet openings ineach series), or more preferably with a radially enlarged peripheral diehaving only a single series of die openings.

2. Description of the Prior Art

U.S. Pat. No. 5,694,833 (incorporated by reference herein) describes asignificant breakthrough in the extruder art. Specifically, that patentdiscloses relatively short length extruders (having an L/D ratio of upto about 6) which are operated at high screw rotational speeds typicallyin excess of 500 rpm. It has been found that extruders of this type arecapable of producing, at relatively high output rates, feed products ofvery high integrity. Moreover, such extruders can be produced at lowercosts as compared with conventional cooking extruders.

Extruders of this type are heretofore have been equipped withconventional plate-type face dies bolted across the outlet end of theextruder barrel. Dies of this variety have a series of die outletopenings which are essentially parallel with the longitudinal axis ofthe extruder barrel. Face dies, while in widespread use, tend torestrict extruder throughputs. Furthermore, because such dies mustwithstand considerable pressure, they are normally relatively expensive.

It has also been known in the past to equip certain long-barrelextruders with so-called “peripheral” dies. Dies of this character havedie outlet openings oriented transverse to the longitudinal axis of theextruder barrel. Generally speaking, peripheral dies have a tendency toadversely affect product quality, particularly where relatively denseextrudates are desired. Accordingly, peripheral dies have not achievedwide spread use in the extrusion art.

SUMMARY OF THE INVENTION

The present invention provides improved extrusion devices of theshort-barrel, high-speed variety through the use of peripheral diestructures. Broadly speaking, extrusion devices of the invention includean elongated, tubular barrel having a material inlet and a spacedmaterial outlet with an inner surface defining an elongated bore; anelongated, helically flighted screw assembly is positioned within thebore and is coupled to a drive for rotating the screw assembly at arotational speed of at least about 500 rpm. A peripheral extrusion dieis disposed across a material outlet of the barrel and has a series ofextrusion outlet openings oriented transverse to longitudinal axis ofthe barrel. Normally, the extruder would have an L/D ratio ashereinafter defined of up to about 7.

In preferred forms of the invention, the extruder barrel has an internalbore of generally frustoconical configuration for at least about 50% ofthe length of the bore from the inlet to the outlet. Usually, the borehas a general frustoconical shape substantially its full length from thebarrel inlet to the outlet so as to present a generally decreasingcross-section of area along the length of the bore. Although twin screwextruders can be used in the invention, generally the screw assembliesare of the single screw variety. In terms of screw rotational speed, thepreferred levels are from about 600-1500 rpm.

One peripheral die assembly in accordance of the invention makes use ofa plurality of series of extrusion outlet openings, with the respectiveseries being axially spaced apart. In such a design, the number ofoutlet openings in each series is different, with the smallest number ofopenings in the series nearest the extruder barrel outlet. In anotherpreferred design, the peripheral die is substantially radially enlargedas compared with the outlet end of the extruder barrel. This allowsprovision of a substantially greater number of die outlet openings, sothat only a single series of such openings is needed to provide therequisite extruder throughput.

In order to achieve a smooth material flow transition from the extruderbarrel outlet to the peripheral die openings, internal flow-directingstructure is normally used with the peripheral dies of the invention.Such flow-directing structure cooperates with the other die componentsfor evenly and smoothly directing material from the extruder barrel tothe peripheral die outlet openings without undue buildup of pressurewithin the die.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary vertical sectional view illustrating a preferredperipheral die assembly mounted on a short length tapered extrusioncooking device;

FIG. 2 is a vertical sectional view taken along line 2—2 of FIG. 1 andillustrating the initial row of peripheral die openings provided in theperipheral die assembly;

FIG. 3 is a vertical sectional view taken along line 3—3 of FIG. 1 andillustrating the second row of peripheral die openings provided in theperipheral die assembly;

FIG. 4 is a vertical sectional view taken along line 4—4 of FIG. 1 andillustrating the third row of peripheral die openings provided in theperipheral die assembly;

FIG. 5 is a vertical sectional view taken along line 5—5 of FIG. 1 andillustrating the fourth row of peripheral die openings provided in theperipheral die assembly;

FIG. 6 is a fragmentary sectional view similar to that of FIG. 1 butillustrating a radially enlarged peripheral die having a modifiedinternal flow-directing bullet and a single row of peripheral dieopenings; and

FIG. 7 is a vertical sectional view of a complete extrusion apparatus inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, a short length extruder assembly 10 isillustrated in FIG. 7. Broadly speaking, the assembly 10 includes apreconditioner 12 and an extruder 14. The latter includes an elongatedtubular barrel 16 having an inlet 18 and an endmost, aperturedperipheral extrusion die 20 across the barrel outlet 21. An elongated,flighted, axially rotatable screw assembly 22 is disposed within barrel16 along the length thereof.

In more detail, the preconditioner 12 is designed to initiallymoisturize and partially precook dry ingredients prior to passagethereof as a dough or the like into the inlet 18 of extruder 14. To thisend, the preconditioner 12 is typically in the form of an elongatedchamber equipped with rotatable internal paddles as well as injectionports for water and/or steam. A variety of preconditioners may be usedin the context of the invention. However, it is particularly preferredto use Wenger DDC preconditioners of the type described in U.S. Pat. No.4,752,139, incorporated by reference herein.

In the embodiment illustrated, the barrel 16 is made up of three axiallyaligned and interconnected tubular head sections, namely inlet head 24and second and third sections 26, 28. The inlet bead 24 is configured topresent the upwardly opening extruder inlet 18 and is positioned beneaththe outlet of preconditioner 12 as shown. In addition, the inlet bead 24has an apertured end wall 30 equipped with seals 32 for engaging sealblock 34. The screw assembly 22 is mounted on hexagonal drive shaft 36and is rotated via schematically depicted conventional bearing housing39 and electric motor 39a.

The second head 26 includes a tubular metallic section 38 equipped withan external jacket 40. The latter has an inlet 42 and an outlet 44 so asto permit introduction of heating or cooling media (e.g., cold water orsteam) into the jacket, thus allowing indirect temperature control forthe head 26. The overall head 26 further presents an internal surfacewith helical ribbed sections 46 which defines an internally extendingbore 48. As shown, the diameter of bore 48 decreases between inlet head24 and third head 28.

Third head 28 is similar in many respects to bead 26 and includes atubular section 50 and an outboard jacket 52, the latter equipped withan inlet 54 and outlet 56 for introduction of indirect cooling orheating media. The inner surface 58 of section 50 presents helical ribs60 and defines an axially extending central bore 62. The bore 62decreases in effective diameter between the end of barrel section 28adjacent section 26 and the end of the section 28 proximal to die 20.

Although not shown in detail, it will be appreciated that the headsections 38 and 50 may be provided with removable internal sleeves ifdesired. This is particularly suitable for larger, high capacitymachines where wear considerations are significant.

The screw assembly 22 includes four rotatable elements mounted on acentral shaft and interconnected in an end-to-end relationship. Inparticular, assembly 22 has an inlet screw section 64, a first screwsection 66, a rotatable spacer 68 and a third screw section 70.

The first screw section 66 includes an elongated central shaftpresenting an outer, generally frustoconical surface and outwardlyextending helical flighting 72. It will be seen that the overallconfiguration of the screw section 66 conforms with the decreasingdiameter of bore 48, i.e., the outer periphery of the flighting 72progressively decreases from the inlet end of the screw section 66 tothe outlet end thereof adjacent spacer 68.

The spacer 68 is mounted on the central shaft between the screw sections66,70. The spacer 68 presents a substantially flattened outer surface asshown, which is slightly inboard of the adjacent margins of the screwsections 66,68.

The third screw section 70 is very similar to screw section 66. That is,the section 70 includes an elongated central shaft presenting anoutermost, frustoconical surface and helical fighting 74.

Again referring to FIG. 7, it will be observed that the overall extruderbore defined by the sections 38,50 is of generally frustoconicalconfiguration leading from inlet 18 to die 20, i.e., the barrel borepresents a generally decreasing cross-sectional area along the lengththereof. Moreover, it will be seen that the effective length of theextruder from the remote end of inlet 18 to the end of barrel 16 (shownas dimension “L” in FIG. 7) versus the maximum diameter of the barrelbore (dimension “D” in FIG. 7) is relatively low, and preferably up toabout 7; the more preferred L/D ratio is up to about 6 and mostpreferably from about 3-6. As used herein, “L/D ratio” refers to theratio measured in accordance with the exemplary length and diameterillustrated in FIG. 7.

It has been discovered that the short length extruders of the inventionshould be operated at the relatively high rotation speeds describedabove. Such high speed operation, in conjunction with the otherpreferred configuration details, gives high throughputs together withgood quality of products.

A particular feature of the present invention involves the use ofperipheral dies in conjunction with the short length of the extruders.One such peripheral die 20 is illustrated in FIGS. 1-4 and 7. In detail,the die 20 includes a first tubular section 76 presenting a converging,generally frustoconical input wall 78 terminating at a central,substantially circular flow-restricting opening 80. The section 76 alsohas a substantially frustoconical, diverging output wall 82 downstreamof the opening 80. As shown, the section 76 is affixed to the butt endof the barrel 14 by means of screws 84. Further, the outer surface ofsection 76 remote from barrel 14 is threaded as at 86. In practice, anannular insert 88 is positioned within the opening 80.

The overall die 20 also includes an apertured, annular segment 90 whichis threadably attached to the tubular section 76. The segment 90 isprovided with four axially spaced apart, circumferentially spaced series92-98 of extrusion outlet openings 100. In order to provide essentiallyeven output from the respective outlet openings series 92-98, the numberof outlet openings 100 in each series is different, i.e., the number ofopenings increases in each of the series 92-98, with the first series 92containing the smallest number of openings 100 (see FIGS. 2-4). Theoutboard end of segment 90 is also threaded as at 102, and supports acomplementally threaded cross plate 104. The latter in turn has anoutwardly extending knife shaft 106 as illustrated.

An internal flow-directing bullet 108 is secured to the inner face ofcross plate 104 by means of screw 110. The bullet 108 presents a centralapex and a diverging, frustoconical surface 112 leading to an outwardlyextending and diverging terminal surface 114. As illustrated in FIGS. 1and 7, the spacing between the output wall 82 of section 76 and thesurfaces 112,114 of the bullet 108 defines a flow path 116 serving todirect product from the extruder barrel 14 towards the openings 100.

Referring now to FIG. 6, a second peripheral die 118 is depicted,mounted on the extruder 14 described previously. Broadly speaking, thedie 18 differs from die 20 in that it is substantially radially enlargedand has on ly a single circumferently space die outlet openings 120. Inmore detail, the die 120 includes an initial tubular section 122 havinga frustoconical input surface 124, a central flow-restricting opening126, and a diverging output surface 128. Screws 130 are employed tosecure the section 122 to the butt end of the extruder barrel asillustrated. Moreover, an annular metallic insert 132 is removablypositioned within the opening 126.

An annular, generally L-shaped in cross-section segment 134 isthreadably secured to the outboard end of section 120 and has thecircumferentially spaced series of die outlet openings 120 therein. Thesegment 134 supports a central cross plate 136, the latter having anoutwardly extending knife shaft 138. A flow-directing bullet 140 issecured to the inner face of cross plate 136 and extends outwardly toengage the adjacent inner surface of the segment 134. The bullet 140 hasa central apex and an essentially uniform conical diverging surface 142.The surfaces 142 and 128 cooperatively define an elongated flow path 144leading towards the openings 120 as shown. An advantage of theconstruction shown in FIG. 6 is that, with only a single series of dieoutlet openings 120, there is no problem of differential flow ofmaterial from axially spaced series of die outlet openings, which canresult in extrudate pieces of varying lengths.

In typical operations employing extruders in accordance with theinvention, an edible material to be processed is first formulated andthen preconditioned, followed by passage into and through the shortlength extruder. In the preferred preconditioner, the material ismoisturized and at least partially cooked. Preconditioning is normallycarried out so that the product leaving the preconditioner has a totalmoisture content of from about 15-40% by weight, and more preferablyfrom about 22-28% by weight. The residence time in the preconditioner isusually from about 15-150 seconds, and more preferably from about 90-150seconds; and the maximum temperature in the preconditioner ranges fromabout 55-212° F., and more preferably from about 180-200° F.

During passage through the extruder, the material is subjected toincreasing levels of temperature and shear and is normally fully cookedas it emerges from the extrusion die. Typical residence times of thematerial in the extruder barrel range from about 10-40 seconds, and morepreferably from, about 20-30 seconds. Maximum pressure levels achievedin the extruder barrel are normally from about 150-1000 psi, and morepreferably from about 300-500 psi. The maximum temperature levelachieved in the extruder barrel is from about 220-300° F., and morepreferably from about 230-250° F.

After leaving the extruder barrel, the material encounters theperipheral die assembly. In the preferred die assemblies describedabove, the material leaving the extruder barrel is first compressed anddirected into and through the central flow-restricting opening, whereupon the material passes outwardly through the flow path for ultimateextrusion through the die outlet openings. During such a process, itwill be appreciated that the material is advanced in a directiontransverse (normally essentially perpendicularly) to the longitudinalaxis of the extruder barrel.

The extruders and methods in accordance with the invention areparticularly suited for the preparation of feed products, especiallyanimal feed products. Such products may be of the expanded variety, suchas typical pet foods, or more dense pellet-type products typically fedto pigs. In such uses, the starting materials usually include a highproportion of grain at a level of at least about 40% by weight (e.g.,corn, wheat, soy, milo, oats), and may include fats and other incidentalingredients. Expanded products in accordance with the invention wouldtypically have a final (i.e., after drying) density of from about 15-25kg/m³, whereas denser pellet-type products would normally have a finaldensity of from about 30-50 kg/m³. Broadly, therefore, products of theinvention would have final densities in the range of from about 15-50kg/m³. The products also have a relatively high Pellet Durability Indexrating, normally above about 75, and more preferably above about 85.

Use of the peripheral die structure has been found to significantlyincrease the capacity of the extrusion assembly, as compared with priormachines of this character employing conventional face dies.Additionally, die manufacturer costs are considerably reduced becauseless expensive materials can be employed owning to the considerable hoopstrength obtained through the use of a peripheral die arrangement.Although peripheral dies have been used in the past in connection withthe processing of textured vegetable protein (TVP) products, they havenot been employed in the context of short length, high speed extruders.Generally, when peripheral dies were used for the extrusion of TVPproducts, unsatisfactory low density extrudates were produced. However,it has been surprisingly found that use of peripheral dies in accordancewith the present invention yields relatively high density productshaving very high pellet integrity.

For example, use of peripheral dies with the short length, high speedextruders of the type described in U.S. Pat. No. 5,694,833, as comparedwith use of conventional face dies, gives increased drive feed rates,higher moisture extrudates, greater degrees of cook, higher bulkdensities and essentially equivalent or increased Pellet DurabilityIndex values.

The following examples set forth preferred extrusion apparatus andmethods in accordance with the invention. It is to be understood thatthe invention is not so limited and nothing in the examples should betaken as a limitation upon the overall scope of the invention.

The following examples set forth preferred apparatus and processingtechniques in accordance with the invention. It is to be understood,however, that these examples are provided by way of illustration andnothing therein should be taken as a limitation upon the overall scopeof the invention.

EXAMPLE

In this example, a short length extruder of the type described in U.S.Pat. No. 5,694,833 was equipped with a peripheral die and used toproduce a feed product.

The extruder was of the type illustrated in FIG. 7 and consisted ofthree heads. In particular, the extruder was made up of the followingcomponents (were all parts are identified with Wenger Mfg. Co. PartNos.): extruder barrel—68781-001, 68782-001, 68784-001; screwassembly—68792-001, 68793-001, 68805-023, 68796-001; final die—74002-219NA, 74002-217 NA, 74002-218 NA, 31600-220 NA, 31600-621 BT. A rotatingknife assembly was adjacent the outlet of the die for cutting theextrudate into a convenient size.

The overall assembly also included a Model 7 DDC preconditionermanufactured by Wenger Mfg. Inc. and generally described in U.S. Pat.No. 4,752,139 incorporated by reference herein. The preconditioner wasmounted upstream of the extruder barrel inlet and served to moisten, mixand partially cook the dry ingredients before entry thereof into theextruder barrel. Specifically, the preconditioner employed a standard390 configuration with 60 beaters mounted on each preconditioner shaft.

In the two test runs, the starting recipe was made up of 70.0% by weightcorn, 18.0% by weight soy bean meal, 10.0% by weight wheat middlings, 1%by weight calcium carbonate, and 1.0% by weight salt.

The following table sets forth certain operating conditions for thepreconditioner and the extrusion device in the two test runs; ingeneral, the material is subjected to increasing levels of heat andshear during passage through the extruder and peripheral die.

TABLE 1 RUN RUN #1 #2 RAW MATERIAL INFORMATION: Dry Recipe Moisture % wb 12  12 Dry Recipe Density kg/m³ 593 595 Feed Screw Speed rpm  85 111PRECONDITIONING INFORMATION Preconditioner Speed rpm Steam Flow toPreconditioner kg/hr  80  80 Preconditioner Discharge Temp ° C.  80  80EXTRUSION INFORMATION: Extruder Shaft Seed rpm 1200  1200  Motor Load %—  85 Water Flow to Extruder kg/hr 9.6 — FINAL PRODUCT INFORMATION:Extruder Discharge Moisture % wb  14 15.5 Extruder Discharge Rate kg/hr1036  1409  Extruder Discharge Density kg/m³ 577 593 ExtruderPerformance Stable Stable

After extrusion the product was conventionally dried to a level of belowabout 10% wet basis (Run No. 2 was analyzed and found to have a moisturecontent of 8.2% wb). The product from Run No. 2 was the best, andexhibited a Pellet Durability Index (PDI) of 95, contained 46.37% byweight total starch, 20.75% by weight gelatinized starch and had a 44.7%cook value. As used herein, “pellet durability index” and “PDI” refer toan art recognized durability test described in Feed ManufacturingTechnology IV, American Feed Association, Inc., 1994, pages 121-122 (andreferenced information), incorporated by reference herein. In such adurability test, the durability of pellets obtained immediately aftercooling when the pellets have a temperature within ±10° F. of ambienttemperature. Durability is determined by tumbling a 500 g sample ofpre-sieved pellets (to remove fines) for 5 minutes at 50 rpm in adust-tight 12″×12″×5″ enclosure equipped with a 2″×9″ internal plateaffixed symmetrically along a 9″ side to a diagonal of one 12″×12″dimension of the enclosure. The enclosure is rotated about an axisperpendicular to and centered on the 12″ sides thereof. After tumbling,fines are removed by screening, and the pellet sample is reweighed.Pellet durability is defined as:

durability=weight of pellets after tumbling/weight of pellets beforetumbling×100

We claim:
 1. An extrusion device comprising: an elongated tubular barrelhaving a material inlet and a spaced material outlet and presenting aninner surface defining an elongated bore; an elongated, helicallyflighted screw assembly positioned within said bore; a drive for axiallyrotating said screw assembly at a rotational speed of at least about 500rpm; and a peripheral extrusion die assembly disposed across saidmaterial outlet and having a series of extrusion outlet openingsoriented transverse to the longitudinal axis of said barrel, said dieassembly comprising a first tubular section adjacent said materialoutlet and presenting a converging input wall, a restricted throughopening, and a diverging output wall downstream of said input wall andopening, said opening having a diameter less than the diameter of saidbarrel, flow-directing structure spaced downstream of said output walland having a diverging outer surface, and annular die plate disposedabout said flow-directing structure and having said outlet openingstherethrough.
 2. The device of claim 1, said die presenting a pluralityof series of said extrusion outlet openings, with the respective seriesthereof being axially spaced.
 3. The device of claim 1, said extruderhaving L/D ratio of up to about
 7. 4. The device of claim 3, said ratiobeing up to about
 6. 5. The device of claim 4, said ratio being fromabout 3-6.
 6. The device of claim 1, said bore being of generallyfrustoconical configuration for at least about 50% of the length of thebore from said inlet to said outlet.
 7. The device of claim 6, said borebeing of generally frustoconical configuration from said inlet to saidoutlet to present a generally decreasing cross-sectional area along thelength of the bore.
 8. The device of claim 1, said screw assemblycomprising a single elongated, axially flighted screw within saidbarrel.
 9. The device of claim 1, said peripheral die having a diametersubstantially greater than the diameter of said barrel at said outletend, with said openings located radially outwardly from said barrel. 10.The device of claim 1, said drive operable for rotating said screwassembly at a rotational speed of about 600-1500 rpm.
 11. The device ofclaim 1, said inner bore-defining surface of said barrel presenting aseries of spaced, helical rib sections along the length thereof.
 12. Amethod of extrusion cooking comprising the steps of: passing anextrudable mixture into the inlet of an elongated extruder having abarrel equipped with an outlet and an endmost peripheral extrusion dieon said barrel at said outlet, and an internal, axially rotatable,flighted screw assembly within the barrel; and rotating said screwassembly at a speed of at least about 500 rpm for advancing materialfrom said inlet along the length of said barrel and out said outlet andthrough said peripheral extension die, said rotating and advancing stepincluding the steps of first passing said material generally axially outsaid barrel outlet, then causing the material to converge downstreamfrom said outlet and pass through a restricted opening, thereaftercausing the material to diverge downstream from said restricted opening,and finally causing the material to pass from said peripheral die in adirection transverse to the longitudinal axis of said barrel.
 13. Themethod of claim 12, including the step of passing said material througha plurality of series of extrusion outlet openings provided in saidperipheral extrusion die, with the respective series thereof beingaxially spaced.
 14. The method of claim 12, said extruder having L/Dratio of up to about
 7. 15. The method of claim 14, said ratio being upto about
 6. 16. The method of claim 15, said ratio being from about 3-6.17. The method of claim 12, including the step of rotating of said screwassembly at a rotational speed of about 600-1500 rpm.